In recent years, a computer with a new concept of being wearable like clothes and able to be operated and used in this state is drawing attention. This computer is called a wearable computer, and is realized based on small and high-performance personal digital assistants.
Progressive researches are also conducted on a technique of carrying out data communications between plural wearable computers via parts of a human body such as arms, shoulders, and bodies. This technique is already proposed in patent literatures and the like (for example, see Japanese Patent Application Laid-Open No. 2001-352298 (pages 4 to 5, FIGS. 1 to 5)). FIG. 1 shows an image of carrying out communications between plural wearable computers via a human body. As shown in FIG. 1, a wearable computer 1 and a transceiver 3′ that is brought into contact with the wearable computer 1 constitute one set. A set of a wearable computer 1 and a transceiver 3′ can carry out data communications with other set of a wearable computer 1 and a transceiver 3′, via a human body. The wearable computer 1 can also carry out data communications with other set of a personal computer (PC) 5 which is other than the wearable computer 1 mounted on the human body and a transceiver 3′a installed on a wall or the like, or a set of the PC 5 and a transceiver 3′b installed on a floor or the like. In this case, the PC 5 is not brought into contact with the transceivers 3′a and 3′b unlike the wearable computer 1 and the transceiver 3′, but is connected via a cable 4, to the transceivers 3′a and 3′b. 
Regarding the data communications via a human body, a signal detection technique according to an electro-optic method using a laser light and an electro-optic crystal is utilized. With this arrangement, an electric field based on information (data) to be transmitted is induced in a human body (i.e., electric field transmission medium), and information based on the electric field induced in the human body is received, thereby achieving transmission and reception of the information. The technique of data communications via the human body is explained in detail with reference to FIG. 2.
FIG. 2 is an overall configuration diagram of a transceiver main body 30′ that is used to carry out data communications via a human body 100. As shown in FIG. 2, the transceiver main body 30′ is used in a state of being in contact with the human body 100 via a transmitting and receiving electrode 105′ and an insulating film 107′ The transceiver main body 30′ receives data supplied from the wearable computer 1 via an I/O (input/output) circuit 101, and transmits the data to a transmitter 103. The transmitter 103 induces an electric field in the human body 100 as an electric field transmission medium from the transmitting and receiving electrode 105′ via the insulating film 107′. The transmitter 103 transmits this electric field to other transceiver 3′ mounted on other part of the human body 100 via the human body 100.
In the transceiver main body 30′, the transmitting and receiving electrode 105′ receives an electric field induced in the human body 100 and transmitted from a separate transceiver 3′ mounted on other part of the human body 100 via the insulating film 107′. An electric field sensor unit 110′ that constitutes an electric field sensor device 115′ applies the received electric field to the above electro-optic crystal, thereby generating a polarization change and an intensity change in the laser light. A light receiving circuit 152′ that constitutes the electric field sensor device 115′ receives the polarization-changed and intensity-changed laser light, and converts the laser light into an electric signal, and processes this electric signal such as amplifies this electric signal. A signal processing circuit 116 that constitutes a receiver circuit 113 removes a frequency component other than a frequency component concerning reception information as the electric field to be detected out of electric signals of various frequencies (i.e., extracts only the frequency component concerning the reception information) with a band pass filter that constitutes the signal processing circuit 116, thereby removing noise from the electric signal. A waveform shaping circuit 117 that constitutes the receiver circuit 113 shapes the waveform (i.e., carries out a signal processing) of the electric signal that passes the signal processing circuit 116, and supplies the waveform-shaped electric signal to the wearable computer 1 via the input/output circuit 101.
As shown in FIG. 3, the electrode can be divided into two for transmission and for reception, respectively. In other words, the transmitter 103 induces an electric field in the human body 100 as an electric field transmission medium from a transmitting electrode 105′a via an insulating film 107′a. On the other hand, a receiving electrode 105′b receives an electric field induced in the human body 100 and transmitted from a separate transceiver 3′ mounted on other part of the human body 100 via an insulating film 107′b. Other configurations and their operation are similar to those shown in FIG. 2.
For example, as shown in FIG. 1, the wearable computer 1 mounted on the right arm makes the transceiver 3′ induce an electric signal concerning transmission data as an electric field in the human body 100 as an electric field transmission medium, and transmit the electric field to other parts of the human body 100 as shown by a wavy line. On the other hand, the wearable computer 1 mounted on the left arm can make the transceiver 3′ receive the electric field transmitted from the human body 100, return the electric field to the electric signal, and receive reception data.
The computer such as the wearable computer 1 and a personal digital assistant such as a portable telephone need to be compact considering convenience of mounting on the human body 100 and carrying as shown in FIG. 1.
However, along miniaturization of the computer and the personal digital assistant, input of information to the computer and the personal digital assistant become difficult.
On the other hand, the electric field sensor unit 110′ in the transceiver main body 30′ includes ones which convert the polarization change of laser light into the intensity change like a polarization modulator, and ones which directly convert the intensity change of the laser light like optical intensity modulators such as an electroabsorption (EA) optical intensity modulator and a Mach-Zehnder optical intensity modulator.
An electric field sensor unit 110′a and a light receiving circuit 152′a that use a polarization modulator 123 is explained with reference to FIG. 4 and FIG. 5, and then, an electric field sensor unit 110′b and a light receiving circuit 152′b that use an optical intensity modulator 124 is explained with reference to FIG. 6 to FIG. 8.
First, as shown in FIG. 4, the electric field sensor unit 110′a using the polarization modulator 123 includes a current source 119, a laser diode 121, a lens 133, the polarization modulator 123 such as an electro-optic element (electro-optic crystal), a first and a second wave plates 135 and 137, a polarizing beam splitter 139′, and a first and a second lenses 141a and 141b. 
The light receiving circuit 152′a includes a first photodiode 143a, a first load resistor 145a, a first constant voltage source 147a, a second photodiode 143b, a second load resistor 145b, a second constant voltage source 147b, and a differential amplifier 112.
Of the above, the polarization modulator 123 has sensitivity in only the electric field that is coupled in a direction perpendicular to a proceeding direction of laser light that is emitted from the laser diode 121. The intensity of the electric field changes optical characteristic, that is, a birefringence index, of the polarization modulator 123. The change of the birefringence index of the polarization modulator 123 changes the polarization of the laser light. A first electrode 125 and a second electrode 127 are provided on both side surfaces of the polarization modulator 123, that are opposite in a vertical direction in the drawing. The first electrode 125 and the second electrode 127 face each other perpendicular to the proceeding direction of the laser light from the laser diode 121 in the polarization modulator 123, and can couple the electric field with the laser light at a right angle.
The electric field sensor unit 110′a is connected to the receiving electrode 105′b via the first electrode 125. The second electrode 127 that is opposite to the first electrode 125 is connected to a ground electrode 131, and functions as a ground electrode to the first electrode 125. The receiving electrode 105′b detects an electric field that is transmitted after being induced in the human body 100, transmits this electric field to the first electrode 125, and can couple the electric field with the polarization modulator 123 via the first electrode 125.
With this arrangement, the laser light output from the laser diode 121 according to the current control from the current source 119 is made parallel light via the lens 133. The first wave plate 135 adjusts the polarization state of the parallel laser light, and inputs the laser light to the polarization modulator 123. The laser light that is incident to the polarization modulator 123 is propagated between the first and the second electrodes 125 and 127 within the polarization modulator 123. During the propagation of the laser light, the receiving electrode 105′b detects the electric field that is transmitted after being induced in the human body 100, and couples this electric field with the polarization modulator 123 via the first electrode 125. Then, the electric field is formed from the first electrode 125 toward the second electrode 127 connected to the ground electrode 131. Since the electric field is perpendicular to the proceeding direction of the laser light that is incident from the laser diode 121 to the polarization modulator 123, the birefringence index as the optical characteristic of the polarization modulator 123 changes, and the polarization of the laser light changes accordingly.
The second wave plate 137 adjusts the polarization state of the laser light of which polarization is changed by the electric field from the first electrode 125 in the polarization modulator 123, and inputs the laser light to the polarizing beam splitter 139′. The polarizing beam splitter 139′ separates the laser light input from the second wave plate 137, into a P wave and an S wave, and converts the laser light into optical intensity change. The first and the second lenses 141a and 141b condense respectively the laser light that is separated into the P wave component and the S wave component by the polarizing beam splitter 139′. The first and the second photodiodes 143a and 143b that constitute photoelectric converting means receive the laser light, convert the P wave light signal and the S wave light signal into electric signals respectively, and output the electric signals. The first load resistor 145a, the first constant voltage source 147a, the second load resistor 145b, and the second constant voltage source 147b convert the current signals output from the first and the second photodiodes 143a and 143b, into voltage signals. The differential amplifier 112 can extract a voltage signal (intensity modulation signal) concerning reception information by differential. The extracted voltage signal is supplied to the signal processing circuit 116 shown in FIG. 2 and FIG. 3.
As shown in FIG. 5, the phase of a voltage signal Sa according to the first photodiode 143a and the phase of a voltage signal Sb according to the second photodiode 143b are deviated by 180 degrees. Therefore, the differential amplifier 112 amplifies the signal component of the opposite phase, and subtracts and removes noise of the in-phase laser light.
The signal processing circuit 116 shown in FIG. 2 and FIG. 3 removes noise from the signal. The waveform shaping circuit 117 shapes the waveform of the signal, and supplies the signal to the wearable computer 1 via the input/output circuit 101.
The electric field sensor unit 110′b and the light receiving circuit 152′b that use the optical intensity modulator 124 is explained with reference to FIG. 6 to FIG. 8. Constituent parts identical with those of the electric field sensor unit 110′a and the light receiving circuit 152′a that use the polarization modulator 123 are assigned with the same reference numerals.
As shown in FIG. 6, the electric field sensor unit 110′b that uses the optical intensity modulator 124 includes the current source 119, the laser diode 121, the lens 133, the optical intensity modulator 124 such as an electroabsorption (EA) optical intensity modulator and a Mach-Zehnder optical intensity modulator, and the lens 141.
The light receiving circuit 152′b includes the photodiode 143, the load resistor 145, the constant voltage source 147, and a (single) amplifier 118.
The optical intensity modulator 124 is configured to change the optical intensity of the light that passes due to the intensity of the coupled electric field. The first electrode 125 and the second electrode 127 are provided on both side surfaces of the optical intensity modulator 124, that are opposite in a vertical direction in the drawing. The first electrode 125 and the second electrode 127 face each other perpendicular to the proceeding direction of the laser light from the laser diode 121 in the optical intensity modulator 124, and can couple the electric field with the laser light at a right angle.
The electric field sensor unit 110′b is connected to the receiving electrode 105′b via the first electrode 125. The second electrode 127 that is opposite to the first electrode 125 is connected to the ground electrode 131, and functions as a ground electrode to the first electrode 125. The receiving electrode 105′b detects an electric field that is transmitted after being induced in the human body 100, transmits this electric field to the first electrode 125, and can couple the electric field with the optical intensity modulator 124 via the first electrode 125.
An electroabsorption (EA) optical intensity modulator 124a as one example of the optical intensity modulator 124 is briefly explained with reference to FIG. 7.
As shown in FIG. 7, when laser light having constant optical intensity is input, the electroabsorption optical intensity modulator 124a varies the optical intensity according to the detection signal concerning the electric field with the constant optical intensity as the maximum. In other words, the intensity of the input laser light is attenuated based on the detection signal concerning the electric field.
A Mach-Zehnder optical intensity modulator 124b as one example of the optical intensity modulator 124 is briefly explained with reference to FIG. 8.
As shown in FIG. 8, the Mach-Zehnder optical intensity modulator 124b has two waveguides 203a and 203b having light refraction indexes different from that of a substrate 201 formed on the substrate 201, thereby confining laser light input via a lens 205 within the waveguides 203a and 203b and branching the laser light. The first electrode 125 and the second electrode 127 apply an electric field to one of the branched laser lights and couple the electric field with the laser light. Thereafter, the Mach-Zehnder optical intensity modulator 124b emits the laser light via the lens 207. When the electric field is applied to one of the laser lights, the phase of this laser light can be slightly delayed or advanced from that of the laser light which is not applied with the electric field.
Referring back to FIG. 6, the laser light output from the laser diode 121 based on the current control by the current source 119 is made parallel light via the lens 133. The parallel laser light is incident to the optical intensity modulator 124. The laser light that is incident to the optical intensity modulator 124 is propagated between the first and the second electrodes 125 and 127 within the optical intensity modulator 124. During the propagation of the laser light, the receiving electrode 105′b detects the electric field that is transmitted after being induced in the human body 100 as explained above, and couples this electric field with the optical intensity modulator 124 via the first electrode 125. Then, the electric field is formed from the first electrode 125 toward the second electrode 127 connected to the ground electrode 131. Based on this coupling of the electric field, laser light of changed optical intensity is emitted. The photodiode 143 of the light receiving circuit 152′b receives the laser light via the lens 141. As a result, the photodiode 143 converts the laser light into a current signal according to the optical intensity of the laser light. The load resistor 145 and the constant voltage source 147 convert the current signal output from the photodiode 143 into a voltage signal, and output this voltage signal. The output voltage signal is amplified by the amplifier 118, and is supplied to the signal processing circuit 116 shown in FIG. 2 and FIG. 3.
The signal processing circuit 116 shown in FIG. 2 and FIG. 3 remove noise. The waveform shaping circuit 117 shapes the waveform, and supplies the signal to the wearable computer 1 via the input/output circuit 101.
However, the optical intensity modulator 124 shown in FIG. 6 cannot extract the intensity modulation signal by differential as shown in FIG. 5, unlike the polarization modulator 123 shown in FIG. 4 that converts the polarization change of the laser light into the intensity change. Therefore, the optical intensity modulator 124 cannot carry out a differential detection. When the photodiode 143 directly receives the output from the optical intensity modulator 124 without carrying out a differential detection, noise of the laser light cannot be removed, which results in poor S/N ratio of the reception signal and degradation of communication quality.
A human hand (human body 100) may hold a set of the transceiver 3′ and the wearable computer 1, as shown in FIG. 9. The transceiver 3′ shown in FIG. 9 has such a configuration that the transceiver main body 30′ is attached to the bottom of the internal wall surface of the insulating case 33 configured by an insulator, and a battery 6 that drives the transceiver main body 30′ is attached on the upper surface of the transceiver main body 30′. Further, the transmitting and receiving electrode 105′ is attached to the bottom of the external wall surface of the insulating case 33, and this transmitting and receiving electrode 105′ is covered with the insulating film 107′. Parts other than the operation/input surface of the wearable computer 1 are covered with an insulating case 11.
However, when the hand holds the transceiver 3′ as shown in FIG. 9, even when an electric field E1 for transmission is induced in the human hand (human body 100) from the transmitting and receiving electrode 105′, electric fields E2′ and E3′ thereof return from the hand to the transceiver 3′ via the side surface of the insulating case 33. Therefore, the transceiver 3′ does not carry out normal transmission operation.